Compound Interest Calculator

What Is Compound Interest?

Compound interestis interest earned on both your initial principal and the interest that accumulates over time. It's often called "interest on interest," and it's the most powerful force in long-term investing. Unlike simple interest, which only grows your original deposit, compounding makes your money grow exponentially — slowly at first, then dramatically faster as the years pass.

The Compound Interest Formula

For a single deposit with no ongoing contributions:

A = P × (1 + r/n)nt

• A = final amount
• P = principal (initial investment)
• r = annual interest rate (decimal)
• n = times compounded per year
• t = years

This calculator assumes monthly compounding and lets you add monthly contributions, which are calculated using the future value of an annuity formula.

The Power of Starting Early

Time matters more than the amount you invest. Consider two savers:

• Saver A invests $200/month from age 25 to 35 (10 years, $24,000 total), then stops. At 7% return, by age 65 they have ~$245,000.
• Saver B waits until age 35, then invests $200/month for 30 years until age 65 ($72,000 total). At 7% return, by age 65 they have ~$245,000.

Despite investing one-third as much money, Saver A ends up roughly even — because their money had 30 extra years to compound.

Realistic Return Rates

Common reference rates for long-term investing:

• S&P 500 historical average: ~10% per year (before inflation)
• After inflation ("real" return): ~7% per year
• Bonds: ~4-5% per year
• High-yield savings: ~4-5% (varies with interest rates)
• Conservative mix (stocks + bonds): ~6-7% per year

Use 7% for stock-market-based long-term investing. Past performance doesn't guarantee future results, and returns vary year to year — but compounding works the same regardless.

How to Maximize Compound Growth

1. Start now. Every year you wait costs you exponentially more than the amount you could have invested.
2. Invest consistently. Monthly contributions automate growth and smooth out market volatility.
3. Reinvest dividends. Don't take the cash — let it compound.
4. Keep fees low. Index funds with 0.03% expense ratios beat actively managed funds with 1%+ fees over the long run.
5. Be patient. The biggest gains come in the later years. Don't panic-sell during downturns.

The Rule of 72

A quick mental shortcut: divide 72 by your annual return rate to estimate how many years it takes to double your money. At 7%, money doubles in about 72 ÷ 7 ≈ 10.3 years. At 10%, it doubles in 7.2 years. This calculator gives you the exact number.